a) Field of the Invention
The invention is directed to an arrangement for the suppression of unwanted spectral components (‘out-of-band’ radiation, as it is called) in a plasma-based radiation source, particularly for the suppression of infrared (IR), visible (VIS), ultraviolet (UV) and/or deep ultraviolet (DUV) spectral components which are typically generated along with short-wavelength radiation, e.g. X-radiation to EUV radiation.
b) Description of the Related Art
For applications in semiconductor lithography, semiconductor wafers require increasingly higher EUV outputs to be generated by plasma-based radiation sources. However, the output of DUV, UV, VIS and IR wavelengths which is emitted by the plasma in addition to the desired EUV radiation is ten-times or multiple-times higher than the output in the desired EUV wavelength band (12-14 nm).
Further, the mirror optics used for EUV lithography reflect particularly UV, VIS and IR radiation appreciably better (˜90%) than the desired EUV wavelengths (˜65%). Since there are approximately ten mirrors contained in EUV lithography systems, this reduces the proportion of EUV radiation transferred to the wafer even more drastically. Further, a thin radiation-sensitive film (comprising a resist) which is provided on the wafer for the lithography process for generating the desired semiconductor structures is not only sensitive to the desired EUV radiation, but also partially absorbs the UV, VIS and IR radiation. Therefore, the resist is heated and expands. Of course, UV and DUV spectral components also especially influence the resist optically, which impairs the high exposure accuracy needed in EUV lithography. Therefore, it is important to filter out as much of the output of unwanted wavelength components (out-of-band radiation) as possible before reaching the wafer.
On one hand, out-of-band radiation is emitted directly by the plasma. On the other hand, it is also emitted by the components for generating the plasma, e.g., by the electrode system (when using a gas discharge plasma) or by a debris filter which is installed very near the plasma in order to intercept fast particles and materials which can condense at room temperature and is heated by the plasma (and possibly by hot generated components). Therefore, every hot object in the EUV radiation path is a source of out-of-band radiation.
Because of the short relaxation time of plasma, the radiation of the very hot plasma is generated in a pulsed manner with a pulse length of less than one microsecond, while the VIS radiation and IR radiation are emitted almost continuously. This latter results from the fact that the hot generated components in the vicinity of the plasma emit continuous IR radiation because of their long cooling period which is much longer than the time between two pulses. Only the fluctuations caused by the pulsed plasma corresponding to the source frequency are superimposed on this continuous radiation.
Many different solutions are known from the prior art for suppressing out-of-band radiation. One of the oldest solutions is the use of filter foils. In this solution, a spectral bandpass filter based on a thin foil window is used at the output of the EUV radiation source. However, because of the high power density of the EUV radiation at high repetition frequency, this foil is exposed to very high thermal loads and high-energy particles from the plasma and therefore to the risk of uncontrolled destruction. This is normally countered by permanently advancing a band-shaped filter. A disadvantage of this filter principle consists in that the potential risk of destruction of the filter membrane is only reduced but cannot be eliminated assuming that the filter band is advanced only very slowly or partially at long intervals for cost reasons.
Further, the publication US 2002/0097385 A1 describes a lithographic projection device in which a grating spectral filter is used for filtering the EUV light. The filter is a reflection grating (echelon grating) for grazing incidence which preferably comprises a material that is transparent to the desired EUV radiation. In a further development according to US 2004/0109149 A1, additional cooling is provided at the back of the substrate of the diffraction grating, and combinations with an upstream gas flow are provided for debris mitigation.
US 2004/0051954 A1 likewise discloses a spectral filter in the form of a reflection diffraction grating. The diffraction gratings are generated by anisotropic etching techniques in a silicon substrate as smooth, flat facets which are defined by (111) crystallographic planes of the substrate.
The solutions mentioned above are disadvantageous primarily because of the highly accurate manufacturing requirements and adjusting requirements for the diffraction gratings to achieve the desired filtration and the high susceptibility of the surfaces to contamination, e.g., by carbon deposits.